1. Field of the Invention
The present invention relates to a color correction table forming method, a color correction table forming apparatus, a control program for controlling the color correction table forming method and apparatus, and a storage medium storing the control program.
2. Related Background Art
Generally, in a computer system or the like, when an image displayed on a monitor is printed and output by a printer, since a color reproduction range of the monitor is remarkably different from that of the printer, a so-called color matching process is necessary to adjust a monitor-displayed color and a printed color so that appearances of these colors become equivalent to each other. As the color matching process, for example, a color correction process to perform an interpolation operation by referring to a color correction look-up table (hereinafter, called a “color correction table”) taking into consideration color characteristics of the monitor and the printer is known.
However, in a process to form the color correction table, various noise can arise and come to be mixed in the values of the formed color correction table. For example, when a colorimetry process is generally performed in order to learn the actual characteristics of the printer, a colorimetry error in this process might cause noise. In addition, a quantization error or the like in a case of quantizing the result of a calculation process might cause such noise. Thus, if noise which has occurred due to such a cause as these comes to be mixed into the values of the color correction table, the change of the values of the table becomes unsmooth. As a result, a tone change (or a gradation change) becomes unsmooth in the image after the color correction is performed, and thus a problem of a pseudo-contour or the like easily arises when the color-corrected image is printed by the printer.
Therefore, in order to eliminate the noise in the color correction table, a smoothing process using a shift unchanged filter is conventionally performed on the values of the color correction table.
However, if the same smoothing process is performed on all the values of the color correction table as in such a conventional technique, the value in the area where the smoothing is not as necessary as elsewhere changes unintendedly from the value it had before smoothing. As an example of an area where smoothing is not so necessary in a color correction table, is an area where it is considered that the value has already changed smoothly before the smoothing process is performed, or an area where it is intended to suppress a change of the value due to the smoothing process for colorimetric reasons or reasons of subjective judgment. Moreover, if it is considered, based on the image printed by the printer, that it is desirable preponderantly to perform the smoothing process only on a specific area on color space, it is conventionally difficult to achieve the smoothing process only for this area. Thus, the smoothness by the smoothing process can-not be changed for each area, whereby it is impossible to form the color correction table for obtaining an image having the desired satisfactory quality for a user.
Moreover, in image equipment such as an ink-jet printer or the like, in a case where R (red), G (green) and B (blue) signals input from an image input unit are converted into device colors (e.g., C (cyan), M (magenta), Y (yellow) and K (black)) of the image equipment and a color image is then output from an image output unit, color reproducibility should be satisfactorily maintained. As a technique to achieve this, a method of performing former-stage color conversion (i.e., color correction) and a latter-stage color conversion (i.e., color separation) by using a three-dimensional LUT (look-up table) is known.
In the former-stage color conversion, so-called color reproduction space mapping is used to compress the range incapable of being color-reproduced by the image output unit and map the compressed range into the range capable of being color-reproduced by the image output unit.
Conventionally, the following technique to perform the color reproduction space mapping technique is known. That is, in a case of performing the color conversion by using the three-dimensional LUT and then performing the color reproduction space mapping (i.e., color area conversion), an output color space (i.e., a space formed by color signals between a color reproduction conversion system and an output system) is divided into an area on which the color reproduction space mapping process is performed and an area on which the color reproduction space mapping process is not performed, the corresponding area is mapped to an input color space (i.e., a space formed by color signals between an input system and the color reproduction conversion system), a and clipping area is discriminated based on the mapped area, thus completing the color reproduction space mapping.
In the latter-stage color conversion, a so-called color separation process is performed to convert the R, G and B signals input from the image input unit into the device colors of the printer.
Incidentally, in the above color conversion method using the LUT, it is often the case that the respective values at lattice points on the LUT are calculated independently, and thus there is no continuity between the values of the adjacent lattice points, whereby discontinuity in regard to the signal values might arise on the LUT. For example, in Japanese Patent Application Laid-Open No. 2001-16476, smoothing by using a three-dimensional filter is performed on R, G and B channels independently in order to eliminate discontinuity. Moreover, in order to prevent gray from becoming a chromatic color as a result of the smoothing process, the area to be processed is divided into a gray area and a chromatic color area, and the smoothing process is performed on the gray area with the constraint of maintaining the relation R=G=B.
However, if the above conventional smoothing process is applied to the former-stage color conversion, the smoothing is performed uniformly on the entire color space other than the gray area(s), and on the gray area(s). Thus, for example, it is impossible to effect precision effects such as locally not performing the smoothing (or performing it in a particular way locally) on a specific chromatic color so that for example a color around a skin color is preserved in the smoothing, and as a result it is necessary to perform an adjustment which considers a color change due to the smoothing before the color adjustment is actually performed. Therefore, there is a problem that it is difficult for the user to perform the color adjustment as a whole.
Moreover, if the above conventional smoothing process is applied to the former-stage color conversion, the processing of the gray area is entirely performed independently of that of the chromatic color area. Therefore, there can occur a problem that a color change is discontinuous between a gray line and an achromatic color in the vicinity of the gray line.
Moreover, if the above conventional smoothing process is applied to one or both of the former-stage color conversion and the latter-stage color conversion, there is a problem that a predetermined value can-not be preserved in the smoothing in a portion which is different from noise that is the target of the smoothing in the vicinity of, e.g., a solid color portion (satisfying the maximum saturation or the maximum density for each color) in the former stage or the latter stage (e.g., the portion which is the peak of a color signal change).